1. Field of the Invention
The invention is directed to a hydraulic control mechanism for actuating a hydraulic actuator using the output of a fixed displacement pump and holding the hydraulic actuator in a selected position using the output of a variable displacement pump.
2. Description of the Prior Art
Work vehicles are provided with working implements that are manipulated by hydraulic cylinders or actuators. With a grapple skidder, the working implement is a pair of tongs that are manipulated by one or more hydraulic actuators. The tongs are supported from a grapple arch which may be coupled to a pivoting boom. Both the grapple arch and boom are provided with hydraulic actuators for controlling the position of the grapple relative to the skidder.
The 648D grapple skidder, manufactured and marketed by the assignee of the present application, is similar to the skidder described above in that it is provided with a grapple arch and boom for positioning the grapple. A single hydraulic actuator manipulates the grapple. This actuator is hydraulically coupled through a control valve assembly to a variable displacement pump. The variable displacement pump directs hydraulic fluid as needed to the grapple.